This invention relates to electrical protection assemblies.
The invention is more particularly concerned with protection assemblies adapted to prevent damage to associated equipment by high power, transient electrical signals, such as caused by lightning strikes and nuclear electromagnetic pulses (NEMP).
Many components of aircraft, and other vehicle, bodies, previously made of metal, are now being replaced by composite materials, such as carbon fibre reinforced materials. These materials can have great advantages of high strength and low weight but, because they are, in general, electrically non-conductive, they do not provide any significant electrical screening as is conventionally provided by metal panels and the like. This reduction in electrical screening can make electrical equipment in the vehicles more prone to interference produced by external electromagnetic noise. Furthermore, a lightning strike or NEMP can induce a high power signal in the equipment, or in cables connecting equipment, causing damage to it, with possible catastrophic consequences.
Low power electrical interference induced in cables can be reduced readily by filtering, either at the connection of the cable to the equipment casing or within the casing. Examples of such filtering is described in, for example, GB No. 2201050A.
The removal of high power pulses caused by lightning or NEMP causes significantly greater problems, especially where it is necessary to provide electrical protection for each of many conductors connected to a multi-pin connector. A gas-filled tube can be connected between the conductor and ground, the tube becoming conductive by ionisation of the enclosed gas when a high voltage pulse is applied to it. This can be effective against one component of the energy in a lightning strike, commonly known as the slow pulses of around 100 microseconds width but is of little use against the other component, known as the fast pulse which can be around 2 microseconds wide, because the gas does not ionise sufficiently in this time to provide significant conduction. For these faster pulses it is necessary to use voltage-dependent resistance means such as a zener diode, varistor or Tranzorb device which conduct more quickly but which are not rated for as high power dissipation as gas-filled tubes. To give effective protection, therefore, it is necessary to use a parallel arrangement of a gas-filled tube and one of these faster responding devices. Careful matching is necessary to ensure that the gas-filled tube takes over the major part of the power dissipation from the parallel device before the parallel device is damaged by excessive power dissipation through it.
Previously proposed devices for multi-pin connectors have employed a respective gas-filled tube and zener diode, or the like, connected in parallel between each pin and ground. This has lead to a connector of great complexity, bulk, weight and cost. The gas-filled tubes, in particular, tend to be relatively bulky. In most multi-pin connector systems it is physically impossible to accommodate individual conductor protectors which have sufficient energy absorption capability for lightning strikes. Furthermore, the demands upon such built-in lightning arrestors have grown by a factor of ten because of the reduction iniscreening and the increased sensitivity of modern electronic systems.